Piezoelectric transformer discharge lamp operating circuit with duty cycle dimming circuit

Abstract

A circuit arrangement for operating a lamp comprising input terminals for connection to a supply voltage source. A DC-AC converter is coupled to said input terminals for generating a high frequency signal. A piezoelectric transformer having a primary side that is coupled to the DC-AC converter and a secondary side that is coupled to terminals for lamp connection and a dimming circuit for adjusting the light output of the lamp. The DC-AC converter generates a substantially square wave shaped signal and the dimming circuit is coupled to the DC-AC converter for adjusting the lamp light output by adjusting the duty cycle of the substantially square wave shaped signal. A lamp operated by the circuit arrangement is dimmable over a wide range while the amount of power dissipated in the circuit arrangement is relatively low, even when the lamp is dimmed.

What is claimed is: 
     
       1. A circuit arrangement for operating a discharge lamp comprising: input terminals for connection to a supply voltage source,   a DC-AC-converter coupled to said input terminals for generating a high frequency signal,   a piezo-electric transformer having a primary side that is coupled to the DC-AC-converter and a secondary side that is coupled to terminals for lamp connection, and   means for adjusting the light output of the lamp, characterized in that the DC-AC-converter comprises a first circuit part for generating a substantially square wave shaped signal and the means for adjusting the light output of the lamp comprises a dimming circuit coupled to the first circuit part for adjusting the duty cycle of the substantially square wave shaped signal.     
     
     
       2. A circuit arrangement according to claim 1, wherein the first circuit part comprises an integrated circuit. 
     
     
       3. A circuit arrangement according to claim 2, wherein the DC-AC-converter comprises an amplifier coupled between the first circuit part and the primary side of the piezo-electric transformer for amplifying the substantially square wave shaped signal to a substantially square wave shaped voltage. 
     
     
       4. A circuit arrangement according to claim 3, wherein the amplifier comprises a series arrangement of inductive means and switching means and the switching means are shunt with the primary side of the piezo-electric transformer. 
     
     
       5. A circuit arrangement according to one or more of the previous claims, comprising a second circuit part for adjusting the frequency of the substantially square wave shaped signal dependent upon the resonance frequency of the piezo-electric transformer. 
     
     
       6. A circuit arrangement according to claim 5, wherein the second circuit part comprises a phase locked loop coupled to the secondary side of the piezo-electric transformer and to the first circuit part. 
     
     
       7. A circuit arrangement according to claim 5, wherein the second circuit part comprises an amplitude locked loop coupled to the secondary side of the piezo-electric transformer and to the first circuit part. 
     
     
       8. A circuit arrangement according to claim 7 wherein the amplitude locked loop samples a voltage present at the secondary side of the piezoelectric transformer and, via the first circuit part, adjusts the frequency of the substantially square wave shaped signal so that the amplitude of the voltage at the secondary side of the piezoelectric transformer is a maximum. 
     
     
       9. A compact discharge lamp comprising: a light source provided with a vessel which is closed in a gastight manner and transmissive for visible radiation,   a housing connected to the light source and provided with a lamp cap,   ballast means electrically connected to the light source for operating the light source and positioned at least partly in a space surrounded by the housing, characterized in that the ballast means comprises a circuit arrangement as claimed in claim 1 in which the DC/AC converter applies a square wave voltage to the primary side of the piezo-electric transformer.     
     
     
       10. A liquid crystal display comprising a backlight equipped with a lamp and ballast means for operating the lamp, characterized in that the ballast means comprises a circuit arrangement according to claim 1. 
     
     
       11. A circuit arrangement according to claim 1 wherein the DC/AC converter comprises an amplifier coupled between the first circuit part and the primary side of the piezoelectric transformer for amplifying the substantially square wave shaped signal to a substantially square wave shaped voltage. 
     
     
       12. A circuit arrangement according to claim 1 further comprising: a series circuit including inductive means and a controlled semiconductor switching means coupled across said input terminals,   means coupling a control electrode of the semiconductor switching means to an output terminal of the first circuit part which supplies said substantially square wave shaped signal, and   means coupling the primary side of the piezoelectric transformer to a junction point in said series circuit between the inductive means and the semiconductor switching means.   
     
     
       13. A circuit arrangement according to claim 2 wherein the supply voltage source comprises a source of DC voltage, the circuit arrangement further comprising: a first variable resistor coupled to said input terminals and to a first input of the integrated circuit so as to adjust the frequency of the generated substantially square wave shaped signal which is developed within the integrated circuit,   an inductor and a semiconductor switching means serially connected to said input-terminals and further coupled to the primary side of the piezoelectric transformer,   a second variable resistor coupled to said input terminals and to a second input of the integrated circuit so as to adjust the duty cycle of the generated substantially square wave shaped signal, and   means coupling a control electrode of the semiconductor switching means to an output terminal of the integrated circuit and which supplies said substantially square wave shaped signal.   
     
     
       14. A circuit arrangement according to claim 13 further comprising: a first capacitor connected in a series circuit with the first variable resistor to said input terminals, and   a second capacitor connected in a series circuit with the second variable resistor to said input terminals.   
     
     
       15. A circuit arrangement according to claim 1 further comprising: a series circuit including inductive means and a controlled semiconductor switching means coupled across said input terminals,   means coupling a control electrode of the semiconductor switching means to an output terminal of the first circuit part which supplies said substantially square wave shaped signal,   second means coupling the semiconductor switching means across the primary side of the piezoelectric transformer, and   a second circuit part coupled to the secondary side of the piezoelectric transformer and to the first circuit part for adjusting the frequency of the substantially square wave shaped signal dependent upon the resonant frequency of the piezoelectric transformer.   
     
     
       16. A circuit arrangement according to claim 1 wherein the first harmonic of the square wave signal is approximately equal to the resonant frequency of the piezoelectric transformer. 
     
     
       17. A circuit arrangement according to claim 1 further comprising: an amplifier coupled to the first circuit part and which comprises a series arrangement of inductive means and switching means and with the switching means in shunt with the primary side of the piezoelectric transformer, and   the inductive means are connected in series with the primary side of the piezoelectric transformer.   
     
     
       18. A circuit arrangement according to claim 1 wherein the dimming circuit includes a variable resistor that controls the duty cycle of the substantially square wave shaped signal independently of the lamp voltage or current. 
     
     
       19. The circuit arrangement as claimed in claim 1 wherein the first circuit part comprises a single integrated circuit that adjusts the frequency and duty cycle of the substantially square wave shaped signal as a function of the supply voltage at said input terminals. 
     
     
       20. The circuit arrangement as claimed in claim 1 wherein the DCAC converter includes a controlled switching device that applies a square wave voltage to the primary side of the piezo-electric transformer. 
     
     
       21. A circuit arrangement for dimming a discharge lamp comprising: input terminals for connection to a supply voltage source,   a DC-AC converter coupled to said input terminals and including a controlled switch for generating a high frequency square wave voltage,   a piezo-electric transformer having a primary side that is coupled to the DC-AC converter to receive said square wave voltage and a secondary side coupled to terminals for lamp connection, and   means for adjusting the light output of the lamp which includes means coupled to the input terminals for adjusting the frequency and duty cycle of the square wave voltage as a function of a supply voltage at said input terminals.